Recent work has established that RadLV/VL3, a highly leukemogenic virus of strain C57BL/Ka mice, contains two viral genomic RNAs, 8,000 and 5,600 bases in length, whereas BL/Ka (B), a nonleukemogenic virus from the same strain, contains only a single genomic RNA, 8,000 bases in length, with about 70% homology to the 8.0 kb genome of RadLV/VL3. We plan to continue the characterization of the RadLV/VL3 genome and the analysis of its expression in established thymic lymphoma cell lines.